Method of and means for indicating condition of memory element and selectron



June 8, 1948. J, RAJCHMAN 2,442,985 v METHOD OF AND MEANS FOR INDICATINGCONDITION OF MEMORY ELEMENT AND SELEG'IRON.

Filed Jan. 8, 1947 i- Ii $252125 5. F l 49 u V 1' f1 0 i 7 ,J

Zhwentor z/m AK a cbau Patented June 8, 1948 r 2,442,985 METHOD or ANDMEANS FOR mmon'rme CONDITION OF MEMORY AND SELECTRON Jan A. Raichman,Princeton, N. 3., assignor to Radio Corporation of America, acorporation of Delaware Application January 8, 1947, Serial No. 720,876

producing a voltage or current which indicates to which one of the twopossible conditions the element had previously been set.

The principle b means of which the individual memory element isprecisely and accurately selected is fully described in theaforementioned copending application. In brief, the system utilizes oneor more grid networks, each of which comprises a plurality of parallelspaced wires adapted to be individually biased with suitable controlpotential. The networks are positioned so as to form a grid mesh.Electrons from a cathode source are directed toward the grid mesh, andby the suitable application of control potentials to adjacent pairs ofwires the electrons are permitted to pass through selected windowsdefined by the intersecting grid wires and to impinge on a targetelectrode. The target electrode consists of dielectric secondary-emissivmaterial, such as mica, and the small area which the electrons strike onpassing through the grid window constitutes a memory element which maybe conditioned to one or the other of two predetermined stableconditions so as to retain or store information indicative of thecondition to which it was previously set. A signal plate is providedwhich is capacitively coupled to all points in the dielectric surfaceand may be formed. for example, by depositing on the rear surface of themica dielectric a thin, transparent coating of metal. Willemite or otherfluorescent material may also be coated on the target so as to producelight when the memory element is struck by electrons. The details ofconstruction and the nature of the operation of the memory elementitself is described and claimed in a copending application of R. L.Snygder, Serial No. 516,425, filed December 31, 194

As discussed more fully in the first mentioned copending application,when a small area or memory element of the secondary-electron emissivedielectric surface is bombarded by electrons having sufficient energy torelease more secondary 9 Claims. (01. 250-415) electrons than the numberof bombarded electrons, (that is, when the secondary emission ratio isgreater than unity) and when there is a secondary-electron collectorelectrode adjacent the memory element, the surface of the dielectricwill assume the potential of the collector. This will then be a stablecondition, since if the surface potential tends to rise above that ofthe collector electrode the secondary emission will be suppressed. Sincea greater number of negative electrons remain on the dielectric element,it will have a lower P tential. On the other hand, if the dielectricelement tends to go below collector potential, the secondary emissionwill tend'to increase, due to the greater collecting field, and the lossof additional electrons will tend to raise its potential. Collectorpotential is, therefore, a stable condition.

If, however, the intensity of electron bombardment is reduced to a valuesuch that th secondary emission ratio becomes less than unity, then thedielectric element will immediately go to cathode potential. This isalso a stable condition, since, if, for any reason the dielectricpotential tends to rise above cathode potential the surface willimmediately attract a large number of negative electrons which willdrive the potential downward. The dielectric element cannot go belowcathode potential as a result of electron bombardment, since thenegative potential would prevent electrons from reaching its surface.

Because of the above described characteristic of the storage or memoryelement, the device has great utility in connection with computingequipment, where it is desired to record and retain for subsequent usevoltages indicative of one or the other of the two digits of the binarysystem of counting or the two on-off-signals coding any other systemsuch as decimal. Thus the condition of stability when the dielectric isat collector potential may be used to represent the digit 1, while thecondition when the dielectric element is at cathode potential may beusedto represent the digit 0 in the binary system (or simply on-oiiconditions). It should also be noted that having once established thedesired condition in a given memory element. the condition may bemaintained indefinitely by continuousl bombarding all of the memoryelement, since, as stated above, the two conditions are stable underelectron bombardment.

The method of conditioning the selected memory elements to a desired oneof the two possible stable potentials is described in the firstmentioned copendine application. In brief, the storage element isconditioned by applying a voltage pulse to the signal plate andcontrolling the electron bombardment of the selected element so as tobring the element to collector or cathode potential, as the case may be.Thereafter the electron current will hold the potential of the elementat the desired value.

The earlier method for deriving information from the storage deviceafter it has been conditioned was somewhat complicated and required acircuit which was rather delicate in adjustment. It is therefore theprimary object of this invention to provide an improved method of andmeans for deriving information from a memory or storage tube indicativeof the condition of a selected memory element.

When the dielectric target is coated with fluorescent material, oralternatively, where the fluorescent material is coated directly on atransparent metallic signal plate and itself functions as thedielectric, light will be emitted from the element when it is bombardedby electrons of sufllciently high velocity. However, when the element isat cathode potential the electrons strike the element at such a low rateof speed and in such a small quantity that no light is produced.Consequently, the presence or absence of light is indicative of thecondition of the mem ory element.

It is a further object of this invention to provide a method of andmeans for deriving information from a storage device of the characterdescribed which is responsive to the light emitted from the selectedelement.

Since. in accordance with the present inven-- tion. the condition of thememory element is indicated by means of a light link, it will beappreciated that information may be derived from the selected elementwithout disturbing or'changing the condition of the element. This is adesirable feature. since it is frequently necessary to read or deriveinformation from a storage device more than once.

It is a further object of this invention to derive information as to thecondition of a memory element without altering its condition.

As previously stated. the condition of each memory element may beretained indefinitely at its assigned value by opening all the windowsof the control grid so as to cause electrons to imbinge simultaneouslyon all elements. This may be called the standby condition. In theoperation of the device it is contemplated, therefore, that the tubewill be maintained in a standby condition at all times except when asingle window is opened for the purpose of applying information to orderiving information from the selected memory element. A practicalstorage device may have 4000 or more individual memory elements. If asubstantial number of these are at collector potential it will be apreciated that a large portion or perhaps the entire surface will emit alight during the standby condition. It is a characteristic of mostfluorescent materials that the light is not extinguished instantaneouslyWhen the electron current is cut off, but dies down gradually inaccordance with substantially exponential or hyperbolic curves. Comparedto the amount of light produced by many elements, that emitted by asingle selected element is extremely small. If the light responsiveindicator is to respond accurately to the condition of the singleselected element its response must be delayed after the general electronbombardment has been terminated for the period of time required, underthe given conditions, for the light produced by all the other elementsto die down to a value substantially less than that of one element. Inthe case of a storage device having 4000 elements the total lightemitted must therefore decrease to a value less than /4000 of itsoriginal value before an accurate indication can be made.

It is therefore a. further object of this invention to provide in adevice having a plurality of elements capable of being conditioned toeither one of two conditions, and which emit light in one of saidconditions, an improved method of and means for determining thecondition of a selected one of the elements by producing an electricalcurrent which is dependent upon the light emitted solely from theselected element when the measurement is made.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionboth as to its organization and method of operation, as well asadditional objects and advantages thereof, will best be understood fromthe following description when read in connection with the accompanyingdrawings, in which Figure 1 is a circuit diagram illustrating aembodiment of this invention;

Figure 2 is a circuit diagram of an alternative embodiment employing anelectron multiplier and Figure 3 is a view illustrating an arrangementfor collecting the light emitted from the entire surface of acylindrical target electrode.

Referring to Figure 1, an electronic storage device 5 is shownschematically. This device is preferably a tube of the Selectron typedescribed in the aforementioned copending application of J. A. Rajchman.For convenience the tube is shown schematically and for the purpose ofillustrating the present invention it is assumed that a,singlerectangular target electrode is employed, although the tube may bemade in accordance with all the structural modifications sug-- gested inthe earlier application. The tube contains a cathode I, an acceleratinggrid 9, a vertical grid network ii, only one wire of which is shown, a"horizontal grid network 13, a collecting electrode [5 and a targetelectrode I1. The target electrode may comprise a thin dielectric micasheet on the inner surface of which is deposited a thin layer offluorescent material, and having on its outer surface a thin,transparent layer of conductive metal to which the lead i9 is connected.

For the sake of simplicity, the tube is assumed to have only fourhorizontal and four vertical grid wires, although it is to be understoodthat these may be multiplied to provide the desired number ofmemory'elements in accordance with the aforementioned earlierapplication. Furthermore, the tube may be constructed either as apotential barrier" or deflection device and the construction andarrangement of the auxiliary electrodes may be modified accordingly. Inorder to illustrate the processes of grid control in its simplest form,a plurality of manually operated switches have been shown for connectingthe individual wires of the control grid networks to suitable potentialsfor "opening and closing the selected windows. In the case illustratedthe required potentials are -10 v, and v. which are produced by abattery 2|. This assumes that the tube is of the deflection type. If thepotential barrier type of construction is employed, it

- switch 5| operable between two positions.

5 will be understood that the control potentials will be and -100 v.

By means of two conductors 23 and 25, the control potentials are appliedto the respective contact points of the four vertical selecting switches21 and the four horizontal selecting switches 25. In addition, a switch3| is provided which may be operated so as to connect conductor 25directly to conductor 23, and thus to apply the opening" potential toboth conductors. This switch is employed during standby operation toopen all the windows of the storage device so as to permit electronbombard ent of all memory elements. In the position s own, switch 3|permits the selection of any one of the 16 variable memory elements byapplying the opening potential to two adjacent pairs of horizontal andvertical grid wires. Since the method of grid selection is not a part ofthe present invention, it need not be described in greater detail. It isto be understood, however, that any of the previously described systemsof control may be employed for electrically selecting the desired one ofthe plurality of memory elements, including those described in acopending application of J. A.

Rajchman, Serial No. 702,775, filed October 11, 1946.

The accelerating and collecting electrodes are connected to suitablesources of positive potential provided by a battery 33. The signalplate, which is a part of the target electrode, is con-' nected byconductor l9 to the positive'terminal of' a. battery 35 through a switch31, the negative terminal of the battery being connected to ground. Thelead I9 is also connected to ground through a resistor 39 and acapacitor 4|. The purpose of switch 31 is to apply a pulse to the signalplate to set the potential of the selected memory element at the desiredvalue. The purpose of the capacitor and resistor is to prevent thevoltage pulse from reducing in amplitude too rapidly when switch 31 isopened.

Associated with the target electrode is a phototube 40. It may bedesirable to focus the target electrode on the phototube by means of alens The photo anode is connected to ground through an output resistor45 across which may be connected any suitable indicator such as a meter41. The photo cathode is connected through a resistor 49 to the movablearm oIrh a e photo cathode is also connected directly to the movable armof an associated switch 53 also having two positions. One contact pointof switch 5| is connected to a source of negative potential produced bybattery 55, while the oppov site contact of the other switch 53 isconnected to a suitable source of positive potential. The movablecontact arms of switches 5| and 53 are operatively connected togetherfor concurrent 0p eration and also are similarly connected to the thebinary system of charging the dielectric surface to collector potential.Switches 21 and 29 are first placed in the position required to selectthe element which is to be read. Switch 3| is' then moved from thestandby position" to the select position, switches 5| and 53 beingsimulthose elements some or all oi which may have previously beenconditioned to collector potential.

It should be noted that the previous position of switch 53, duringstandby, desensitized the phototube since a positive potential wasapplied to the photo cathode of the tube, which potential also chargedcapacitor 51 to the same value. The operation of the combined switchesto the select position then disconnects the positive potentlal andcauses a negative potential to be applied to the photo cathode throughresistor 49. In a time determined by the time constant of the R.-C.network, the photo cathode potential will reduce to the value necessaryto sensitize the photo tube for normal operation. It is Wellknown that aphototube will not function when its cathode is positive with respect toits anode and that a negative cathode potential is required before thetube will operate.

The time constant of the R.-C. network, comprising resistor 49 andcapacitor 51, is selected so as to provide a time delay after theoperation of switches 3|, 5| and 53 sufilcient to hold the phototube ina desensitized condition until after the luminescence of the fluorescentmaterial has decreased to a value less than that produced by the oneselected element. When this time period has elapsed the tube issensitized and will respond to the light emitted from the selectedelement.

If the selected element is at collector potential it will be bombardedby electrons and will produce luminescence. This will cause a current toflow through resistor 45 which will be indicated by meter 41. On theother hand, if the selected element is at cathode potential, noluminescence will be produced, and this condition will be indicated bythe absence of a deflection of meter 41. While the indicator has beenillustrated as a meter, it is to be understood that the outputpotential, or the absence of an output potential, as the case may be,may be employed to actuate other devices which may be used in connectionwith computing apparatus, and that therefore thr term indication is notto be limited to a visual indication, but is to include the productionof any condition indicative of or responsive to the condition of thememory element.

In order to prevent a spurious indication or a spurious operation ofthecontrol device, when the control switches are returned to the standbyposition, switch 53 has been provided for the purpose of applying apositive biasing potential to the photo cathode so as to desensitize thetube instantly. Since this potential is not applied to the cathodethrough the time delay network, the phototube is cut off immediately andwill not respond to the light produced by the other elements when theelectron bombardment is resumed.

An alternative arrangement is shown in Figure 2, in which the storagedevice or selectron 5 has not been shown in detail, but is assumed to beas illustrated in the preceding figure. The only difference between thetwo figures is that the sensitivity of the light responsive device hasbeen increased by utilizing a conventional photo multiplier tube 59which may be of the type commonly known as 931-A or IP21. successivelygreater positive potentials are applied to the nine dynodes by means ofa battery SI and a voltage divider 63 in the conventional manner. Thephoto cathode is connected to switch through resistor 49 as before. Theselect contact of switch 5| is connected to the negative terminal ofbattery BI and, since the first dynode is at a more positive potential,the operating bias is thus supplied. The

cathode is also connected through switch 53 to a potential obtained fromvoltage divider 63 which is more positive than, that of the firstdynode. Thus it will be seen that the operation of the biasing controlsystem under the control of switch El and 53 is the same as thatpreviously described. Output is taken from the anode and may be appliedto the grid of output tube 65.

In accordance with one arrangement described in the copendingapplication Serial No. 665,031, the storage device is provided with acylindrical target electrode. In order to focus the light from allportions of such cylindrical electrode the arrangement illustrated inFigure 3 can be employed.

The storage device or selectron 5 is suitably maintained within alight-tight container 61 so that the base section thereof extendsthrough the center of a parabolic mirror 69. At the focal point ofmirror 69 there is located the light responsive device such as phototube40. In this manner the light produced by the memory elements over theentire cylindrical surface of the target electrode may be focused on thelight responsive device. The circuit arrangements may be as shown in thepreceding figures.

There has thus been described a method of and means for indicating thecondition of a memory element by means of a light link which does notaffect the state of the memory elements and which permits its conditionto be determined independently of the condition of all the otherelements.

What I claim is:

1. In a device having a plurality of elements adapted to emit light asan indication of one condition and to emit no light as an indication ofanother condition, the method of determining the condition of apredetermined one of said elements which comprises extinguishing allelements other than the one to be indicated, producing an electricalcurrent which is dependent upon the light emitted from said element, andproducing a condition responsive to said current.

2. In a device having a plurality of elements capable of beingconditioned to either one of two conditions and which emit light in oneof said two conditions, the method of determining the condition of oneof said elements which comprises extinguishing all elements other thanthe one to be indicated, producing an electrical current which isdependent upon light emitted from said element, and producing acondition responsive to said current.

3. In a device having a plurality of elements adapted to be bombardedselectively with electrons, which are capable of being conditioned toeither one of two conditions. and which emit light under bombardment inonly one of said conditions; the method of determining the existingcondition of one of said elements which comprises discontinuing electronbombardment of all of said elements except the one to be indicated,producing an electrical current responsive to the light emitted fromsaid one element, and producing a condition responsive to said current.

4. In a device having a plurality of elements adapted to be bombardedselectively or simultaneously with electrons, which are capable of beingconditioned to either one of two conditions, and which emit light underbombardment in only one 0! said conditions, the method of determiningthe existing condition of one of said elements which comprisesdiscontinuing electron bombardment of all of said elements except theone to be indicated, producing at a fixed time after aid discontinuancesufficient to insure that the light from any of said elements exceptsaid one element has been extinguished an electrical current responsiveto the light emitted from said one element, and producing a conditionresponsive to said current.

5. In a device having a plurality of conditionable elements adapted tobe bombarded with electrons and which emit light under bombardment whenin one condition but not when in another condition, the combination offirst switching means for causing the selective bombardment of only oneof said elements, light responsive means adapted to be energized bylight emitted from said selected element and means controlled by saidlight responsive means for indicating the condition of said element.

6. In a device having a plurality of conditionable elements adapted toemit light when in one condition and to emit no light when in anothercondition, th combination of switching means for extinguishing all ofsaid elements except a selected one; light responsive means associatedwith said device; means for normally desensitizing said light responsivemeans, and means for sensitizing said light responsive means duringperiods when only said selected element is emitting light.

'7. In a device having a plurality of conditionable elements adapted tobe bombarded with electrons and which emit light under bombardment whenin one condition but not when in another condition, "the combination offirst switchin means for causing the selective bombardment of only oneof said elements, light responsive means associated with said device;second switching means operable in one condition to desensitize saidlight responsive means and in the other condition to sensitize saidlight responsive means, and means operatively coupling said first andsecond switching means whereby said light responsive means is sensitizedonly when said one of said elements is bombarded,

8. A device of the character described in claim 7 which includes, inaddition, delay means for delaying the sensitizing of said lightresponsive means for a predetermined interval after the operation ofsaid first switching means.

9. A device of the character described in claim 7 which includes, inaddition, time delay means, operative upon conditioning said firstswitch to cause the selective bombardment of only one of said elements,for delaying the sensitizing of said light responsive device by a timesufficient to insure that the light emitted by all of said elements.except said selected element,- has been extinguished.

JAN A. RAJCHMAN.

